Lawinenlagebericht

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Lawinengefahr

Thu
Hochgebirge
1 · Gering
Waldgrenze
1 · Gering
Unterhalb
1 · Gering
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Kernpunkte

Large avalanches are unlikely, but you could trigger small avalanches on steep slopes with fresh wind-drifted snow. The drifts are largest and most common at upper elevations in the Soldier Mtns where more snow fell yesterday and is expected today. 

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–1.5

    Gusty winds likely formed thin wind slabs in exposed terrain yesterday. With today's expected snowfall and wind, the slabs should grow and form on more slopes as the day progresses, increasing the odds you'll find them. How will the wind slabs behave today?

    • They formed on a variety of surfaces and could be very sensitive (easy to trigger) on some slopes and stubborn (difficult to trigger) on others. 
    • Where the slabs buried stout crusts or weak snow, avalanches you trigger could be wider than you'd expect. 
    • You may find hard, stiff, and soft wind slabs. The harder/stiffer the slab, the higher the probability it could break above you rather than at your feet or sled.
    • Wind slabs could exist near ridgelines, on the sides of cross-loaded gullies, and in isolated areas lower on slopes. 
    • The relatively small slides could travel very quickly on slick surfaces beneath the new snow. 

    Pay attention to how the snow feels under your skis or boards. If the snow surface suddenly stiffens or becomes more supportable, you've found a wind slab. Cracks shooting out in front of your skis or boards are a red flag warning you to be cautious. Surface clues like dunes or ripples can tell you the wind has been at work.  

    While these avalanches won't be huge, they could be powerful enough to carry you downslope. Think about the consequences of being pushed around by even a small avalanche before committing to skiing or riding in consequential terrain (above cliffs, trees, creeks, etc). 

Avalanche Discussion

After a couple of months of almost no snow, we started seeing some precipitation beginning a week ago. Last week’s warm storm brought enough snow and rain to set off several large slab avalanches near Banner Summit. The culprit was weak snow (facets, facets and crusts, and maybe some surface hoar shards on some slopes?) that formed in January. Additional weak layers formed in February and early March, making the upper snowpack messy and complex. The relative strength of the weak snow, the presence and thickness of crusts or surface hoar, and the characteristics of the slab above the weak snow change rapidly as you move up or down in elevation and around the compass to different aspects. Large slab avalanches are not likely to fail on the upper snowpack persistent weak layers today, but the weak layer and slab spatial variability make it nearly impossible to accurately forecast stability on a given slope.

How do we deal with complex situations like this? KISS: Keep it Simple Stupid/Scotty. The weakest snow in the upper 18” of the snowpack appears to exist on the northern half of the compass (NW-N-NE-E aspects) and spans the middle/upper elevation boundary. Strong winds have prevented widespread weak layers from forming in true alpine terrain, and rain and warm temperatures have “cooked” or melted most potential weak layers at lower elevations. In the “money zone” where the weak snow exists, we (the SAC forecasters) are dialing back our terrain choices. We’re beginning to avoid large or high-consequence starting zones over 35* where the weak layers might exist and a slab has formed in the upper 1-2 feet of the snowpack.  

We’ve seen a handful of interesting avalanches fail on the weak snow in the upper snowpack over the past month. The prudent move is to treat those events as foreshadowing rather than outliers. Each small storm, including today’s, bumps up the chances of avalanches failing on this weak snow and makes me less interested in playing in that terrain until stability/instability patterns emerge.

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